It's largely cost rather than sales; sf stories tended to have specially made costumes based around a standard design, historicals a wider range borrowed from stores. That made the latter cheaper to make at the time, but ironically the former are cheaper to animate.I got my copy of Fury from the deep today, only 15 serials to go.Technically only 14 and Mission to the Unknown but I would like it as its own DVD.
Bummer about the historicals, is there really no market for them? At this point I don't think they would sell fewer copies because lets face it, the only people buying animated reconstructions of 55 year old episodes are completists.